The exact origin of Thomond, originally as an internal part of Munster, is debated. It is generally held that the Déisi Muman pushed north-west starting from the 5th to the early 8th century, taking the area from the Uí Fiachrach Aidhne of the Connachta. Eventually, the Dál gCais rose to power in all of Munster, to the detriment of the Eóganachta. The person most famously associated with this is Brian Bóruma, High King of Ireland, best known for his feats at the Battle of Clontarf. Four generations down the line and after providing three more High Kings, the Dál gCais were unable to hold onto all of Munster and so Thomond came into being as a separate entity.

Between the mid-12th and late 13th century, when much of Ireland came under direct English control and/or settlement, Thomond too came into the Anglo-Irish sphere. The de Clare family established a colony at Bunratty, while the Butler and FitzGerald families also made inroads. However, from the time of the Battle of Dysert O'Dea, Thomond was restored as a kingdom, with its rulers reinstating Limerick within their overrule. Not until the 1540s did the ruling O'Brien dynasty accommodate with English rule.

County Clare was sometimes known as County Thomond in the period immediately after its creation from the District of Thomond.[2]

In 1841, an estimation of the extent of the kingdom was undertaken by John O'Donovan and Eugene Curry[3]

"The principality of Thomond, generally called the Country of the Dal-Cais, comprised the entire of the present Co. of Clare, the Parishes of Iniscaltra and Clonrush in the County of Galway, the entire of Ely O'Carroll, the Baronies of Ikerrin, Upper and Lower Ormond, Owney and Arra, and somewhat more than the western half of the Barony of Clanwilliam in the County of Tipperary; the Baronies of Owenybeg, Coonagh and Clanwilliam, and the eastern halves of the Baronies of Small County and Coshlea in the County of Limerick."[4]

The entire Province of Munster was under the control of the O'Brien (Ua Briain) clan under the leadership of Toirrdelbach Ua Briain and his son Muirchertach from 1072-1114. Their capital was located in Limerick. In a bid to secure the High Kingship of Ireland for the clan, Muirchertach encouraged ecclesiastical reform in 1111 with the creation of territorial dioceses over the entire island. They had support for their bid from several foreign connections including the Norwegian king Magnus Bareleg and the Anglo-Norman baron Arnulf de Montgomery, who were both united to the clan through marriage in 1102.[5] Their claim to the High Kingship was countered by the O'Neill (Uí Néill) clan in Ulster under the leadership of Domnall MacLochlainn of Ailech. Though Muirchertach campaigned hard in the north, he was unable to obtain the submission of Ailech. When he fell ill in 1114 he was deposed by his brother Diarmait. Muirchertach did briefly regain power, but after his death in 1119 his brother's sons took control of the clan.[5]

MacLochlainn's plans to restore the High Kingship to the north was thwarted by his ally Tairrdelbach Ua Conchobair of Connacht who formed an alliance with the O'Brien's. In 1118 Conchobair partitioned Munster between the sons of Diarmait and Tagh Mac Carthaig. The northern section of the province became the O'Brien Kingdom of Thomond (Tuadh Mhumhain "North Munster") and the southern became the Mac CarthaighKingdom of Desmond (Deas Mhumhain "South Munster").[5]

From the 12th to the 14th centuries, the Norman invasion and their multiple attempts to take Thomond from the Gaels was the main challenge to the realm. The picture was complicated by rival branches of the Ó Briain trying to ally with various different Normans to enforce their own line as reigning over Thomond. At the time of the Norman invasion of Ireland in 1169, Domhnall Mór Ó Briain was king of Thomond. Domhnall was a man of realpolitik; his main concern was upholding his position in Thomond and was not against collaborating with Strongbow and others against rival Gaelic kingdoms such as Ossory, Desmond and Connacht.[6] Domhnall even acknowledged Henry II as Lord of Ireland at Cashel in 1171, but a mere two years later when Plantagenet tried to grant Thomond to Philip de Braose this situation was overturned. The Dál gCais defeated a Norman army at the Battle of Thurles in 1174, where over 700 were killed. The following year when Raymond le Gros captured Limerick through a naval invasion, Domhnall re-took it and burned it rather than have it in foreign hands.[nb 1] The twenty years after that for the Gaels of Thomond were more secure.

After the death of Domhnall Mór a period of destructive feuding among his offspring caused a great territorial decline in Thomond. The brothers Muircheartach Finn Ó Briain and Conchobhar Ruadh Ó Briain fought with each other, seeing Muircheartach's reign interrupted between 1198-1203. Muirchertach himself was blinded by the Normans in 1208 and was soon forced into abdication due to no longer being righdamhna. Donnchadh Cairprech Ó Briain had to deal with dissent from the Mac Con Mara and Ó Coinn against his rule, so brought in the Laigin's Mac Gormáin as his standard bearers. Donnchadh also enlisted the support of the de Burgh and other Normans in this fight, which came at a costly price; Limerick and lands surrounding it in what would later become County Limerick and County Tipperary.[nb 2] Under pressure from the Butlers, Thomond was now not too different to what would become County Clare, protected by the River Shannon. Donnchadh moved his capital to Clonroad.

Brian Ruadh Ó Briain kept the pressure up by burning Clare Castle and forcing the de Muscregos' to flee to Wales, but he had troubles from his own countrymen. The Mac Con Mara, Ó Deághaidh and Mac Gormáin clans supported his nephew Toirdelbach Ó Briain, a son of Tadhg Cael Uisce, for the kingship instead and revolted. Seeing an opportunity, Edward II offered Thomond to Thomas de Clare if he could take advantage of the Ó Briain feud. The events which followed have passed down to history as the Cathreim Thoirdhealbhaigh. After Brian briefly regained Clonroad with de Clare's help, Toirdelbach arrived with support from Galway in the form of the de Burghs and once again took control in 1277. Brian was executed by his "ally" at Bunratty, but de Clare was soon reconciled with his son Donnchadh mac Brian Ó Briain and supported him against Toirdelbach. The feud continued until Donnchadh drowned at a party on Islandmagrath, on the River Fergus. With Toirdelbach now undisputed king, the Mac Con Mara were able to hound de Clare.

The last successful attempt by a Norman to play divide and conquer within the Thomond kingship was the case of Maurice FitzGerald, Earl of Desmond. A very powerful man, he was constantly rumoured during his life to have wanted to make himself King of Ireland. He successfully sponsored Brian Bán Ó Briain in overthrowing Diarmuid from Clan Tadhg during 1343-1350, but equally decisive was the sides taken by Mac Con Mara, leading to Diarmuid's restoration. With the exception of a failed Norman attempt to reconstruct Bunratty Castle by Thomas de Rokeby from 1353-1355, the Norman settler-colonialist project in Thomond was at an end until the submission of the Ó Briain in the 16th century. The Norman Lordship was weakened by the Bruce Wars of 1315-1318, the Black Death of 1349-1350 (which disproportionally hit Norman controlled towns) and besides that English forces were more invested with the Hundred Years' War in France and their own internal conflict the Wars of the Roses to focus too much on Ireland. All of these factors allowed for a 15th-century Gaelic resurgence, not only in Thomond but across a significant part of Ireland outside of the Pale.

Brian Bán was the last of Clann Briain Ruadh to hold the kingship and from 1350 onwards, Clann Tadhg held sway. Mathghamhain Maonmhaighe Ó Briain came to power before the death of his uncle Diarmuid and he was named as such because he spent time as a foster child in Máenmaige. His succession was disputed by his uncle and brother; Toirdelbhach Maol Ó Briain and Brian Sreamhach Ó Briain. Of the two Brian Sreamhach gained the upper hand and when his uncle tried to enlist the help of the Earl of Desmond to wrestle back the realm, Brian thoroughly routed them at Croom in a key military success. This had the added benefit of winning back Limerick for Thomond and Sioda Cam Mac Con Mara was placed in the city as a warden in 1369. The friendship with the de Burghs of Galway was maintained by Brian and when Richard II was at Waterford in 1399, he paid nominal homage and was well received. Thomond was now in such a position that Conchobhar mac Mathghamhna Ó Briain's twenty six-year reign was marked as a time of peace and plenty.

The years 1426-1459 were marked by a succession of three sons of Brian Sreamhach reigning; Tadhg an Glemore Ó Briain, Mathghamhain Dall Ó Briain and Toirdelbhach Bóg Ó Briain. During this time, Mathghamhain Dall was deposed by his brother Toirdelbhach with the familiar military assistance of the de Burghs (whom he had formed a marriage alliance with). Greater things were to come from the ascent of Toirdelbhach's son Tadhg an Chomhaid Ó Briain. From Inchiquin, Tadhg took advantage of the Wars of the Roses, forming an alliance with the Ó Néill in 1464. He managed to ride south through Desmond (the rest of the old Munster) and enforce the cíos dubh on the Anglo-Normans. This was a kind of Gaelic pizzo which Tadgh's great-grandfather Mathghamhain Maonmhaighe had first been able to enforce as a price of protection. His military prowess was such that the Earl of Desmond was forced to give back to Thomond what would later become County Limerick. Mac Fhirbhisigh hints that the men of Leinster planned to raise Tadhg to the High Kingship of Ireland before his death and claims he was the greatest Ó Briain since Brian Bóruma himself.

Thomond was wealthy in the 15th century; Domhnall Mac Gormáin (died 1484) was described as the richest man in Ireland in terms of live stock. During the reign of Conchobhar na Srona Ó Briain, Thomond maintained alliances with the Mac William Uachtar of Clanricarde and the Butlers. In the latter case, they were opposing the Kildare FitzGeralds, earning the ire of Gerald FitzGerald, Earl of Kildare who had earned the favour of the new Tudor king Henry VII as Lord Deputy of Ireland. Despite Kildare's fearsome reputation, Conchobhar met him in battle at Ballyhickey, near Quin, in 1496 and was successful in turning him back. Toirdelbhach Donn Ó Briain as part of his pact with Ulick Fionn Burke took part in the Battle of Knockdoe in 1504; along with the Mac Con Mara and Ó Briain Ara; against the Earl of Kildare, which they lost. The struggle had been started by a feud between de Burgh and the Ui Maine. Ó Briain later defeated Kildare at Moin na Brathair, near Limerick. Thomond intended to support the Ó Néill against the Ó Domhnaill in a northern feud, but by the time Ó Briain arrived, it was over. Toirdelbhach's life came to an end trying to defend the Ó Cearbhaill of Éile from the Earl of Ormond at Camus, near Cashel; he died "by the shot of a ball."

Although the Silken Thomas issue was resolved by late 1535, Thomond had marked itself out by providing refuge to enemies of the Crown of England in Ireland. The English forces had in turn enlisted the services of Conchobhar's own son Donnchadh Ó Briain who had cemented an alliance with the Butler family by marrying the daughter of the Earl of Ormond. According to Butler, Donnchadh pledged to help them conquer Thomond, aid English colonisation, adopt English laws and help them take over Carrigogunnell Castle. This castle was a symbol of Gaelic defiance, as it had remained out of Anglo-Norman hands for over 200 years. When the castle was attacked by Grey, it surrendered due to Donnchadh. With the loss of east Thomond and the destruction of O'Brien's Bridge, Thomond was in a lot of trouble. Conchobhar, along with loyal supporters such as the Mac Con Mara, continued to fight on and managed to conclude a truce with Grey in 1537.

^The psychological effect that this gesture had on the Normans is evident from when Philip de Braose, Robert FitzStephen and Miles de Cogan rode out to take Limerick city and fled in a great panic upon seeing it in flames. Realising that Domhnall Mór and the Dál gCais would sooner burn it to the ground than have anyone but themselves rule it.

^The Normans also attempted to takeover the key religious post of Bishop of Killaloe. Geoffrey de Marisco promoted his nephew Robert Travers into the post in 1215, with support of the English monarchy. The Gaels disputed his election to Pope Honorius III and in this case Rome took their side, deposing Travers in 1221 in favour of Domhnall Ó hÉanna.